Weeds are a continual threat to the yield of grain crops throughout the world because they compete with the stop for water, sunlight and nutrients. In many parts of the world, herbicides are heavily relied upon to control weeds. Herbicides have facilitated agricultural systems based on minimum tillage (soil disturbance) resulting in reduced soil erosion, evaporation and improved crop yields. However, the heavy reliance on herbicides has led to the evolution of herbicide resistant weeds, which now threaten global food production.
Researchers have found that one way to gain control of herbicide resistant weeds is to remove the weed's seeds from the field before the seeds shed and land on the soil. Removing the weed's seeds, interrupts the reproduction of plants, resistant or not resistant, and thus controls the evolution of herbicide resistant weeds. During grain harvest there is an opportunity to collect weed seeds using a combine harvester and thus control herbicide resistant weeds. Weed seeds above the cut height of the combine harvester enter the harvester and are processed. The weed seeds are removed from the plant stem in the combine harvester's thresher and fall onto the combine harvester's cleaning sieve. The size and aerodynamic properties of a weed seed determines its fate in the harvester. Seeds small enough to fit through the harvester sieve that have a terminal velocity similar or higher than the gain fall through the harvester sieve and end up in the grain tank. Seeds that are too large to fall through the sieve or have a terminal velocity lower than the grain are likely to exit the rear of the combine harvester with the residue material (mostly chaff residue material). Commonly, this residue material is collected to remove weed seeds from the field by using a chaff residue material cart, or baler, or simply placing the residue material in a row to be burnt. However, all of these methods remove nutrient rich residues from the field which goes against the principle of retaining maximum possible residue cover for conservation agriculture methods. Furthermore, all of these methods require post-harvest operations, and those are an added cost and time commitment to the farmer.
An attractive alternative to removing or burning residue material to control weed seeds is to devitalize the weed seeds in the field and retain all the residues and hence nutrients in the field. Physically damaging seeds by milling has long been known to devitalize seeds. Technology to process weed seeds with a mill concurrently with grain harvest has long been sought which has resulted in some prior art arrangements.
U.S. Pat. No. 3,448,933 (Roy and Bailey 1969) described a roller shear mill to process weed seeds in the clean grain screenings. However, this approach still allows weed seeds with low terminal velocity to exit with the residue material and be spread by the harvester. Furthermore, newer combine harvesters do not screen the clean grain and so this invention is not applicable for modern combine harvesters. U.S. Pat. No. 5,059,154 (Reyenga 1991) describes using of a pair of rollers to mill chaff residue material at the exit of the harvester sieve. Using a crushing action to damage seeds in a large stream of chaff residue material is problematic as surrounding material can cushion seeds. Another mill was developed to be more effective than roller milling as described in another patent AU 2001038781 (Zani 2001). The mill disclosed in that patent only had the capacity to process a small proportion of the total chaff residue material exiting the harvester sieve. Thus, this invention relied on removing the majority of the chaff residue material with a sieve prior to being processed. The sieve used to separate weed seeds from chaff residue material was not able to handle the increased chaff material residue loads when modern combine harvesters increased in capacity and thus development has not continued.
An alternative approach was adopted for the Harrington Seed Destructor (HSD) U.S. Pat. No. 8,152,610 (Harrington 2012) by processing the entire chaff residue material stream. The HSD uses a cage mill modified from the mining industry to pulverize the chaff residue material and any weed seeds contained within as they are discharged from the combine harvester. Due to the size, weight and power requirement of the cage mill, the HSD is trailer mounted with its own engine towed behind the combine harvester. The chaff residue material and straw is transferred from the combine harvester to the HSD through flexible connections.
The cost and complexity of the HSD is likely to limit its commercial viability. To reduce the cost and complexity of harvest time weed control, it is desirable to have a weed seed devitalization device integrated into a combine harvester capable of dealing with the large volume and rate of discharge of air and chaff residue material entrained with weed seeds exiting the harvester sieve.
The HSD cage mill is not suitable for integration into a combine harvester because:    1) power consumption of the cage mill is such that it requires a separate power source;    2) the large size of the mill is not readily integrated into the already large combine harvester;    3) the complication of a counter rotating cage drive system adds size, weight and complex drive mechanisms;    4) the heavy weight of the cage mill, frame work and drive system would add to the already sizable weight of a combine harvester;    5) the conversion of a wide rectangular feed from the harvester sieve to the small circular feed of the cage mill with the drive shaft going through the inlet, introduces considerable spacing issues if it is to be integrated into a combine harvester.